1. Field of the Invention
The present invention relates to a toner for developing electrostatic charge image which is used in appliances employing electrophotographic processes employed in copiers, printers, facsimiles, and the like, and particularly, color copiers; it also relates to a method for producing the same. The present invention furthermore relates to a developer for electrostatic charge image and to a method for forming images using said toner for developing electrostatic charge image.
2. Description of the Related Art
Methods for visualizing image information via electrostatic charge image, such as an electrophotographic method, are widely used in various fields. In the electrophotographic methods, an electrostatic charge image is formed on the surface of a photosensitive body (latent image retaining body) by charging and light-exposing steps, and the electrostatic latent image is developed by a developer containing a toner, which is then visualized by transferring and fixing steps. There are two-component developer comprising a toner and a carrier, and a one-component developer using a magnetic toner or a non-magnetic toner alone; however, the toners are generally produced by kneading and crushing method comprising melting and kneading a thermoplastic resin together with a pigment, a charge controlling agent, and a releasing agent such as a wax, followed by cooling, finely crushing, and classifying. If necessary, inorganic or organic particles may be added to the surface of the toner particles in order to improve fluidity and cleaning properties of these toners.
Recently, the use of copiers, printers, and multifunction devices of copiers and printers or facsimiles employing color electrographic methods is widespread; however, in order to realize appropriate gloss in reproducing color image or transparency for excellent OHP images, it is generally difficult to use a releasing agent such as wax. Accordingly, large amount of oil has been applied to the fixing roll to assist peeling off the copy image; however, this led to cause sticky feeling on the copy images inclusive of those on OHPs, made it difficult to generate additional writings into the image using pens, and increased heterogeneity in gloss. Furthermore, the application of commonly used polyethylene, polypropylene, or waxes such as paraffin, to the general use black-and-white copies is further difficult because they impair OHP transparency.
Furthermore, if transparency should be sacrificed, for instance, it is next to impossible to suppress the toner exposure to the surface in case the toner production method using the kneading and crushing method known in the art is employed. This leads to problems when used as developers, such as a considerable loss of fluidity, causing filming in developing machines and photoreceptor, and the like.
As a method to fundamentally solve these problems, there is proposed a production method based on polymerization for controlling exposure of the waxes to the surface by internal encapsulation, which comprises dispersing, with a colorant, an oil phase containing the monomers as the starting materials of the resin, and then directly effecting polymerization in an aqueous phase to obtain a toner.
Further, as a means which enables intentionally controlling the toner shape and the surface structure, proposed in JP-A-63-282752 and JP-A-6-250439 are methods of manufacturing toners by means of emulsion polymerization aggregation method. These methods generally comprise preparing a dispersion of resin particles by emulsion polymerization while preparing a colorant dispersion by dispersing a colorant in a medium, and then mixing them together to obtain aggregated bodies of size corresponding to toner particle diameter, followed by heating to obtain a toner by fusion and coalescence.
The methods above not only realize encapsulating waxes, but also easily reduce the diameter of toners, thereby making it possible to reproduce images with higher resolution and clarity.
As described above, in electrophotographic processes, to provide high quality image and to maintain stable toner performance under various mechanical stresses, it is extremely important to not only select the pigment and releasing agents, optimize the quantities and suppress the surface exposure of the releasing agent, but also improve gloss and releasing properties under no fixing oil, while suppressing hot offset by optimizing the resin characteristics.
On the other hand, the development of a technology capable of fixing at lower temperatures is desired in order to reduce energy consumption, and recently, to enhance energy conservation in particular, it is demanded that the power supply to the fixing machine should be cut off when it is out of service. Accordingly, the temperature of the fixing machine should be instantaneously elevated to the operation temperature on switching on for power supply. To that end, it is preferred to reduce the heat capacity of the fixing machine. However, in such case, the temperature of the fixing machine tends to fluctuate too large as compared with the conventional case. In other words, the temperature overshoot becomes too large on applying electric power, and the drop of temperature on feeding paper also becomes large. Furthermore, in case a paper smaller in width than the width of the fixing machine is continuously fed, the temperature difference between the paper feeding part and the part with no paper feeding also becomes large. The phenomenon above is particularly distinctly observed when a high-speed copier or a printer is used, because the power supply tends to be insufficient. Hence, electrophotographic toners capable of fixing at lower temperatures and free of offset generation to high temperature regions, i.e., the so-called toners with wide fixing latitude, are strongly demanded.
As a means for lowering the fixing temperature of the toners, it is known to use polycondensation type crystalline resins exhibiting sharp melting behavior with temperature change for the binder resins constituting the toners. However, crystalline resins have difficulties in crushing, and are generally unfeasible in case melt-kneading and crushing method is employed.
Furthermore, in the polymerization of polycondensation type resins, a reaction lasting for 10 hours or longer under highly reduced pressure and stirring with high power input at high temperatures exceeding over 200° C. is necessary, but this leads to large energy consumption. Moreover, for this purpose, most cases require a huge equipment investment to assure durability of the reaction facilities.
In addition, in case of manufacturing the toner by emulsion polymerization aggregation method as above, the polycondensation type crystalline resin may be polymerized, emulsified in an aqueous medium to obtain latex that is then aggregated with a pigment, wax, and the like, and then fused to coalesce.
However, the emulsification of a polycondensation type resin requires a non-efficient step with high energy consumption, such as emulsifying by applying high shear force under high temperature exceeding 150° C., or dissolving in a solvent and dispersing the low viscosity solution in an aqueous medium, followed by removal of the solvent.
Furthermore, it has been found difficult to overcome the problem of hydrolysis while emulsifying in the aqueous medium, and the unavoidable generation of contingent factors remained in material design.
The problems above are distinct in crystalline resins, but these are also the case with non-crystalline resins.
For instance, in JP-A-2002-351140 is proposed a method for producing a toner for developing electrostatic charge image, which comprises manufacturing a molten body of the toner starting material by heating and melting toner starting materials containing at least a polyester resin, emulsifying the molten body in an aqueous medium to form fine resin particles, aggregating the fine resin particles, and melt-adhering them to obtain an aggregate of the fine resin particles. In this case, a well-known polycondensation catalyst such as tetrabutyl titanate was used as the catalyst for the monomers, for instance, trimellitic anhydride (TMA) as polyfunctional carboxylic acid, terephthalic acid (TPA) and isophthalic acid (IPA) as dicarboxylic acid, polyoxypropylene(2.4)-2,2-bis(4-hydroxyphenyl)propane (BPA-PO) and polyoxyethylene(2.4)-2,2-bis(4-hydroxyphenyl)propane (BPA-EO) as aromatic diols, and ethylene glycol (EG) as aliphatic diol, and the like, and the reaction was carried out at 220° C. for 15 hours under a nitrogen gas flow at an ordinary pressure, thereafter reducing pressure and continuing reaction at 10 mmHg. Thus was obtained polyester having a weight average molecular weight in a range of from about 5,000 to 90,000. Then, the polyester was molten and kneaded with a colorant, wax, and the like, and the resulting melt-kneaded product was fed into a dispersion emulsifier CAVITRON CD1010 (product of EUROTEC, LTD.) after heating to 190° C., to which 0.5 wt. % dilute ammonia water heated to 160° C. using a heat exchanger was added to CAVITRON at a rate of 1 L per minute. After dispersing, the slurry dispersion was cooled to 60° C. and taken out. For preparing the toner, the dispersion must be further agglomerated, fused, rinsed, and dried, but it is clear that that the resin production and the resin emulsification require a large amount of energy, and is therefore practically unfeasible.
Furthermore, emulsification and dispersion under such high energy conditions tend to cause decomposition of resins, and generate uneven distribution of the composition, or makes it difficult to achieve resin particles with uniform particle size distribution in the dispersion. Moreover, practical problems such as unexpected agglomeration of particles and the like occurred during the storage of the dispersion. Concerning the toners using such materials, there are problems, as a matter of course, not only on the initial image quality, but also on the image stability and the like during continuous printing.
Furthermore, recently proposed is a method of preparing polycondensation resins by dispersing polycondensing monomers in water together with a catalyst. As the report on successful polycondensation of polyester in an aqueous medium, there can be mentioned, for instance, U.S. Pat. No. 4,355,154.
However, the invention disclosed in U.S. Pat. No. 4,355,154 is still insufficient for an industrially stable production of polyesters for toner usages, because it is difficult to obtain high molecular weight polymers. The reason why it is difficult to increase the molecular weight of the polyester is, because, it is difficult to shift the equilibrium of polyester synthesis to the products side by accelerating dehydration of the monomer oil droplet dispersed in water.
There are also reports on the synthesis of polycondensation type resin in an organic solvent. For instance, JP-A-10-1536 discloses a method for producing an unsaturated polyester comprising heating an aliphatic alcohol and an aliphatic polybasic acid in an organic solvent at 100 to 200° C. to effect dehydration reaction.
However, the method of the invention disclosed in JP-A-10-1536 cannot avoid the generation of problems concerning the installation of facilities for recovering organic solvents, the environmental impact, and the like. Furthermore, the organic solvents enumerated as preferred ones, such as anisole, phenetol, and diphenyl ether, are far from being general purpose organic solvents, and are deemed as subjects of regulation. Moreover, heating to high temperatures of 150° C. or higher is necessary in case the polyester produced according to the invention is dispersed in water to make particles; this is not preferable from the viewpoint not only of energy consumption, but also of causing unintentional hydrolysis which affects fixing properties. Yet more, the particle size distribution of the resulting dispersed particles spreads to a wide range, which also affects the particle size distribution and compositional distribution of the toners produced from such particles, and made them practically unfeasible. The organic solvent used as the solvent partly remained in the toners, which affected charging and fixing properties, leading to practically unfeasible results.
In addition, in case the crystalline resin was used alone, there has been found a problem that the resulting toner had insufficient mechanical strength and charging properties. In order to solve such problems, there is proposed a toner using both of the crystalline resin and the non-crystalline resin.
For instance, JP-A-2003-50478 discloses a toner for developing electrostatic charge image containing at least a crystalline compound, a binder resin, and a colorant, and this toner for developing electrostatic charge image is characterized in that its differential scanning calorimetry curve as measured by a differential scanning calorimeter (DSC) shows a distinct endothermic peak in the temperature range of from 50 to 100° C. in the first heating process, and that this peak area decreases to ⅓ or less in the second heating process. Furthermore, JP-A-2004-206081 discloses an image forming toner containing at least a thermoplastic resin (A), a colorant (B), a wax (C), and a crystalline polymer (D), which is characterized in that, when measured with a differential scanning calorimeter, one of the DSC endothermic peak temperatures attributed to (C) and (D) shifts to the lower temperature side by 2° C. or more as compared with the DSC endothermic peak temperatures obtained by measuring (C) and (D) alone.
However, although these toners had superior low temperature fixing properties, they still had problems of generating filming on the photoreceptor during the electrophotographic processes, and, particularly, problems have been found on long term sustainability of high quality image under high temperature and high humidity conditions.